The invention relates to translating-type fluid-power actuators wherein a dynamic seal or gland is needed to prevent or minimize pressure fluid leakage at the tail end of an actuator cylinder, namely, the end via which a driven piston rod must longitudinally displace, in the course of actuator operation.
Such actuators are commonly double-acting and are used for actuation of aerodynamic control elements of an aircraft, as for the variable setting of aeleron, rudder, elevator or the like control surfaces.
Dynamic seal action against the piston rod must provide utmost assurance against loss of pressure fluid, under a range of operating and environmental temperatures which can extend from -65.degree. F. to as much as +400.degree. F., with minimum friction and wear of a retained seal element at interface with the displacing piston rod. Elastomeric seal elements have been commonly employed, but under the more severe demands of modern aircraft elastomeric materials have exhibited significant undesired characteristics, notably losing shape and extruding into running clearances, and otherwise developing extremes of friction variation in performance of the involved actuator. Premature seal failure and unpredictable mechanical hysteresis in actuator performance are encountered, all too often.
More particularly, in conventional design, a dynamic seal of the character indicated consists of an elastomeric ring of varying cross-sectional shape, retained in a groove in the tail end of the cylinder, and sealing capability is a function of the modulus and compression of this ring. Thermal compensation for high temperatures is gained by providing a groove width greater than the compressed axial extent of the elastomeric seal member, but sufficient seal squeeze must be designed into the construction, in order to compensate for seal-member contraction at low temperatures. Thus, the seal member can slide or roll in the groove, resulting in seal failure.
To offset such failure, it has been attempted to use another ring, usually of a plastic material, in conjunction with the rubber ring, in which case the plastic ring is in contact with the piston rod, and the rubber ring is used as a static seal and provides a preloading resilient function, forcing the plastic ring against the piston rod. This eliminates the spiral, nibble and wear problems of elastomeric material in rubbing contact with the piston rod, but the thermal problems remain unsolved.
Other dynamic-seal designs seek to avoid use of elastomerics, by employing a spring ring inside a plastic shell, of "C"-shape cross-section. In such designs, the spring may be helical, canted-helical, or characterized by tangs or fingers. Sealing force around the perimeter of the seal cannot be consistent, so that wear is not uniformly distributed, being localized at the respective spaced points of spring, finger or tang contact with the plastic shell. Thermal compensation is gained in the same way as with an "0" ring, but problems remain, in respect of seal-slipping and tilting in the retaining groove.